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Referencing Method of the referent location gmllr:PositionExpression does not have to be the same as the Linear Referencing Method of the position expression containing the distance expression which uses
this referent. There shall be no offset expression in the referent location gmllr:PositionExpression; all referents shall lie on the linear feature.
At least one of the attributes gmllr:position or gmllr:location is usually specified, unless it can be implied. If both are given they shall refer to the same physical location.
The element gmllr:ownedBy specifies the feature which owns the referent. Referents are owned by a single feature. For example, the reference markers along Interstate 95 are owned by the feature which
represents Interstate 95. The referent representing the intersection with First Avenue along Washington Street is owned by Washington Street if it is used to specify relative linearly referenced locations along
Washington Street. The location of this referent will most likely be specified with a position expression along Washington Street. A different referent can represent the intersection with Washington Street along First
Avenue. This referent is owned by First Avenue and is used to specify relative linearly referenced locations along First Avenue.
The gml:AbstractFeature which owns a gmllr:Referent does not have to be the same gml:AbstractFeature which is the gml:LinearElement for the gml:PositionExpression
containing that gmllr:Referent. For example, if US Interstate Highway 95 is coincident with US Federal Route 1, it is possible to specify locations along Route 1 the linear element in the position expression using
referents owned by Interstate 95 reference post 18. In this case, the location of the referent itself most likely would have been made along Interstate 95 as the linear element since the referent is owned by Interstate 95.
9.3.15 ReferentPropertyType
complexType name=ReferentPropertyType sequence minOccurs=0
element ref=gmllr:Referent sequence
attributeGroup ref=gml:AssociationAttributeGroup complexType
A property that has a referent as its value domain may either be an appropriate referent encapsulated in an element of this type or an XLink reference to a remote referent where remote includes referents located
elsewhere in the same document. Either the reference or the contained element shall be given, but neither both nor none.
9.3.16 MeasureType
complexType name=MeasureType simpleContent
extension base=double attribute name=uom type=gml:UomIdentifier
extension simpleContent
complexType gmllr:MeasureType is the same as gml:MeasureType except that the uom value is optional.
9.3.17 LRMNameType
complexType name=LRMNameType annotation
documentation Informative values from ISO 19148 Annex C
documentation
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appinfo restriction base=string
enumeration value=milepoint enumeration value=trueMileage
enumeration value=kilometre-point enumeration value=kilopoint
enumeration value=chainage enumeration value=hectometre-point
enumeration value=reverseMilepoint enumeration value=reverseKilometre-point
enumeration value=milepointWithLateralOffsetsInFeet enumeration value=milepost
enumeration value=kilopost enumeration value=kilometre-post
enumeration value=referencePost enumeration value=countyMilepoint
enumeration value=crossStreet enumeration value=controlSection
enumeration value=percentage enumeration value=normalized
enumeration value=stationing enumeration value=address
enumeration value=mileMeasure enumeration value=kilometreMeasure
restriction appinfo
annotation simpleContent
extension base=gml:CodeType simpleContent
complexType gmllr:LRMNameType is a codelist which includes the informative values of Linear Referencing Method
names from ISO 19148 Annex C and also allows for extensions of user-defined Linear Referencing Method names. Because there is such a diverse range of Linear Referencing Methods and names, this is offered as
informative only. The enumerated names should only be used if their meaning matches that of the so-named Linear Referencing Method in ISODIS 19148, Annex C.
In accordance with Linear Referencing Methods having a gmllr:type of ‘absolute’, measurements are made along the linear element from its start, in the direction of the linear element. Commonly used absolute
Linear Referencing Methods include milepoint also known as true mileage, its metric analogue kilometre- point also known as kilopoint, chainage, hectometer-point, reverse milepoint and its metric equivalent
reverse kilometer-point and milepoint with lateral offsets in feet.
An LRM name of ‘milepoint’ designates a Linear Referencing Method where measurements are made in decimal miles along the linear element from its start, in the direction of the linear element see ISODIS 19148,
C.2.2.
An LRM name of ‘trueMileage’ designates an alternative name for a ‘milepoint’ Linear Referencing Method see ISODIS 19148, C.2.3.
An LRM name of ‘kilometre-point’ designates a Linear Referencing Method which is identical to the ‘milepoint’ Linear Referencing Method except that the gmllr:units is kilometre instead of mile and the
gmllr:distanceAlong is measured in kilometres see ISODIS 19148, C.2.4.
An LRM name of ‘kilopoint’ designates an alternative name for a ‘kilometre-point’ Linear Referencing Method see ISODIS 19148, C.2.4.
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An LRM name of ‘chainage’ designates a Linear Referencing Method which is identical to the ‘milepoint’ Linear Referencing Method except that the gmllr:units is metre instead of mile. Though the original
British definition of chainage might imply measuring in chains, chainage was adopted as a consensus term for this Linear Referencing Method by a team of European Road Authority representatives including British
for ISO 14825 Geographic Data Files GDF. see ISODIS 19148, C.2.5.
An LRM name of ‘hectometre-point’ designates a Linear Referencing Method which is identical to the milepoint LRM except that the gmllr:units is hectometre 100 metres instead of mile and the
gmllr:distanceAlong is measured in hectometres. see ISODIS 19148, C.2.6.
An LRM name of ‘reverseMilepoint’ designates a Linear Referencing Method where measurements are made in decimal miles along the linear element from its end, in the direction opposite to the direction of the linear
element see ISODIS 19148, C.2.7.
An LRM name of ‘reverseKilometre-point’ designates a Linear Referencing Method which is identical to the ‘reverseMilepoint’ Linear Referencing Method except that the gmllr:units is kilometre instead of mile and
the gmllr:distanceAlong is measured in kilometres see ISODIS 19148, C.2.7.
Many organizations establish a topological network of links and nodes and then specify linear locations along uniquely identified links as an absolute distance from the start of the link the begin node location. This
International Standard is very specific about separating the linear element that which is being measured from the Linear Referencing Method how it is measured. Therefore, what is typically referred to as link offset is
actually the application of an absolute Linear Referencing Method milepoint, kilometre-point to link linear elements.
In a topological network, links do not have length and cannot be measured. In order to use a milepoint or kilometre-point Linear Referencing Method then, a gmllr:Curve geometry is typically associated with each
link. Though measurement is actually done along the curve, it is portrayed as being along the associated link. This International Standard also supports the notion of interpolative measurement along a
gmllr:DirectedEdge, so that a percentage or normalized Linear Referencing Method can be used to specify linear locations along a link. see ISODIS 19148, C.2.8.
Therefore, an LRM name of ‘linkOffset’ designating a Linear Referencing Method with a fixed linear element type of link is therefore not included in the enumeration.
An LRM name of ‘milepointWithLateralOffsetsInFeet’ designates a Linear Referencing Method which is an extension of the ‘milepoint’ Linear Referencing Method. A location is first determined by measuring along the
linear element a distance in decimal miles specified by the gmllr:distanceAlong value. Then, if a gmllr:LateralOffsetExpression is specified, this location is adjusted in accordance with the
gmllr:offsetLateralDistance given in feet. The metric analogue would be kilometre-point or hectometre-point with offsets in metres. see ISODIS 19148, C.2.9. Support for these Linear Referencing
Methods requires a Linear Referencing Method With Offsets see 9.5.
In accordance with Linear Referencing Methods having a gmllr:type of ‘relative’, measurements are made along the linear element from a specified from referent location, usually in the direction of the linear element
unless overridden by a towards referent, in which case the direction from the from referent towards the towards referent. Commonly used relative Linear Referencing Methods include milepost, its metric equivalent
kilopost also known as kilometre-post, reference post, County milepoint, cross street, and Control Section.
An LRM name of ‘milepost’ designates a Linear Referencing Method where measurements are made in decimal miles along the linear element from the closest, preceding milepost. Measuring is done in the
direction of increasing mileposts which is usually the direction of the linear element.
In some States in the US, a single set of mileposts are used for both carriageways of a divided highway. Mileposts are usually numbered in the north or east predominant direction of the highway Even if separate
linear elements are defined for each carriageway, the along measuring is always done in the direction of
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increasing mileposts, even though this is opposite the direction of the westbound and southbound carriageway linear elements.
Mileposts are one mile apart. Sometimes reconstruction of a part of a roadway may change its overall length. To maintain the milepost spacing of exactly one mile, all posts within as well as those beyond the area of
reconstruction must be relocated. This is not always feasible or desirable, especially if the road is hundreds of miles in length and the reconstruction occurs near the beginning of the road. Many times the State will only
relocate mileposts within the reconstructed area, prorating the reconstructed distance between the number of mileposts affected. The consequence is that they are no longer one mile apart. This means that the Linear
Referencing Method henceforth becomes a reference post Linear Referencing Method, requiring knowledge of the inter-post spacings.
Mileposts are owned by a gmllr:Feature linear element. The milepost location, if specified, shall be specified using a gmllr:PositionExpression. The linear element of this
gmllr:PositionExpression will be the owning linear element. The Linear Referencing Method of this gmllr:PositionExpression does not have to be milepost. A milepoint Linear Referencing Method would
allow locating the milepost a certain number of miles from the start of the linear element for example, milepost 3 would have a distance along equal to 3.0. Alternatively, a milepost Linear Referencing Method
could be used to specify the location of the milepost relative to the previous one here milepost 3 would have a from referent of milepost 2 and a distance along of 1.0. Since all mileposts would have a distance along of
1.0 from the previous milepost and since they are named by their mileage from the start of the linear element, the location does not have to be persisted; it can be implied milepost 3 is 3.0 miles from the start of the linear
element that owns it. see ISODIS 19148, C.3.2.
An LRM name of ‘kilopost’ designates a Linear Referencing Method which is identical to the ‘milepost’ Linear Referencing Method except that the gmllr:units is kilometre instead of mile and the
gmllr:distanceAlong is measured in kilometres. Kiloposts are exactly one kilometre apart. If not, it is actually a kilometre reference post Linear Referencing Method. Kilopost locations would be specified by using
a metric Linear Referencing Method such as kilopoint or kilopost, or would be implicitly inferred by the kilopost name. see ISODIS 19148, C.3.3.
An LRM name of ‘kilometre-post’ designates an alternative name for a ‘kilopost’ Linear Referencing Method see ISODIS 19148, C.3.4.
An LRM name of ‘referencePost’ designates a Linear Referencing Method which is similar to the milepost Linear Referencing Method except that the reference posts are not necessarily exactly one mile apart. It is
therefore necessary to explicitly specify the location or possibly position of each reference post. This can be done with a milepoint Linear Referencing Method, specifying their absolute distance along from the beginning
of the owning gmllr:Feature. Alternatively, they can be located using a relative reference post Linear Referencing Method, locating each reference post a distance along from the previous reference post. Then, if
reconstruction occurs, only those reference posts in the reconstruction area which are moved would have to have their distance along updated see ISODIS 19148, C.3.5.
An LRM name of ‘countyMilepoint’ designates a Linear Referencing Method which allows the milepoint value to be reset to zero when a highway enters a new County. If the linear element along which locations are to be
linearly referenced is a single highway which traverses through many Counties, then this Linear Referencing Method is used. Linearly referenced locations are specified as a distance in miles from where the highway
enters the County the from referent.
If however, the highway changes its identity when it enters the new County for example, it becomes County Route 42 and locations are to be linearly referenced along Route 42, then Route 42 is the linear element and
a simple milepoint absolute Linear Referencing Method is appropriate, since it already starts at zero at the County line see ISODIS 19148, C.3.6.
An LRM name of ‘crossStreet’ designates a Linear Referencing Method which allows a location to be specified at a distance along a linear element, measured in miles from the intersection of one street with the linear
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element in a direction towards the intersection of another street with the linear element see ISODIS 19148, C.3.7. Support for this Linear Referencing Method requires support for Towards Referents see 9.4.
An LRM name of ‘controlSection’ designates a Linear Referencing Method which allows the measure value to be reset to zero when a highway enters a new Control Section. If the linear element along which locations are
to be linearly referenced is a single highway which traverses through multiple Control Sections that are defined according to agency specific business rules, then this Linear Referencing Method is used. Linearly
referenced locations are specified as a distance in the specified measurement unit from where the highway enters the Control Section the from referent or starting Control Section Anchor Point.
The philosophy behind the establishment of Control Sections is to distinguish between significant differences in road physical characteristics for example, change from an undivided highway section to a divided highway
section or vice versa andor to limit the length of a measured section of road in order to minimize accumulative measure value errors. Control Section beginend points Control Section Anchor Points may be
established either at topological nodes or at points where the linear element intersects a stable administrative boundary for example, a County boundary.
If each Control Section is a separate linear element, then any of the absolute Linear Referencing Methods milepoint, kilometre-point should be used instead see ISODIS 19148, C.3.8.
In accordance with Linear Referencing Methods having a gmllr:type of ‘interpolative’, measurements are interpolated in accordance with the default length of the linear element gmllr:measure. Commonly used
interpolative Linear Referencing Methods include percentage and normalized. Interpolative Linear Referencing Methods are the only ones which are likely to be used for linear elements of type
gmllr:DirectedEdge.
An LRM name of ‘percentage’ designates a Linear Referencing Method where measurements are made along the linear element from its start and are expressed as the percentage that this is of the total length or weight
of the linear element see ISODIS 19148, C.4.2.
An LRM name of ‘normalized’ designates a Linear Referencing Method which is identical to the percentage Linear Referencing Method except that the measured values range from 0 zero to 1 one instead of 0 zero
to 100. A measured value of 0 zero represents a location at the start of the linear element and 1 one represents a location at the end of the linear element.
Two other Linear Referencing Methods commonly used are stationing and address, though neither is addressed in ISODIS 19148.
An LRM name of ‘stationing’ designates the simplest stationing Linear Referencing Method. It has a gmllr:type of absolute and gmllr:units of feet. A plus sign, +, is often interjected between the
hundreds place and the tens place 1+00 is 100 feet. If station equations are supported, the Linear Referencing Method gmllr:name should instead be stationingWithStationEquations, with a gmllr:type
of relative; the location of each station equation should be a gmllr:Referent. There would also be metric equivalents, such as metricStationing with, for example, 1+000 representing 1 kilometre.
An LRM name of ‘address’ designates the simplest addressing Linear Referencing Method. It has a gmllr:type of interpolative. Here the gmllr:distanceAlong would be an address number. This would
be interpolated against the minimum and maximum address numbers held by the linear element typically a block. So, if the block goes from address number 101 through 199, a gmllr:distanceAlong address of
150 would be half way down the block 150-101199-101 = 0.5. More sophisticated address methods might accommodate independent address numbering on each side of the street but might require splitting the
street into two linear elements.
Linear referencing by address is very approximate and only works for simple addressing schemes. Complications such as address numbers containing alpha suffixes, addressing around cul-de-sacs, apartment
complex addressing, addresses which differ by building floor and addresses assigned in date order preclude the use of an address Linear Referencing Method.
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In accordance with Linear Referencing Methods having a gmllr:type of ‘localInterpolative’, measurements are interpolated locally along a segment of a curve type of linear element bounded by two control points
having m coordinate values bracketing the distance along measure value. Commonly used local interpolative Linear Referencing Methods include mile measure and kilometre measure. Support for these Linear
Referencing Methods requires a linear element of type ‘curve’ see 9.3.5.
NOTE Measured M coordinates are described in ISO 19133 Tracking and navigation, sub-clause 6.5
Measured Coordinates. An LRM name of ‘mileMeasure’ designates a Linear Referencing Method where the gmllr:distanceAlong
measurement is expressed as miles along the linear element. The exact location is determined by finding the two control points along the linear element which have M coordinate values which bracket the
gmllr:distanceAlong value and then interpolating between these two points based on the length of the curve segment between them.
An LRM name of ‘kilometreMeasure’ designates a Linear Referencing Method where the gmllr:distanceAlong measurement is expressed as kilometres along the linear element. The exact
location is determined by finding the two control points along the linear element which have M coordinate values which bracket the gmllr:distanceAlong value and then interpolating between these two points
based on the length of the curve segment between them.
Specification of the above defined Linear Referencing Methods is provided in informative Annex A.
9.3.18 LRMTypeType
